JPH0370198B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates generally to nuclear reactors, and more particularly to fuel assemblies for such reactors.

In most nuclear reactors, the reactor core consists of a number of elongated fuel assemblies, and each fuel assembly is held in an organized array by a grid spaced along the fuel assemblies. It consists of multiple fuel rods and control rod guide thimble. The grid is attached to the control rod guide thimble. Upper and lower nozzles at each end of the fuel assembly are secured to a control rod guide thimble that extends upwardly and downwardly over the ends of the fuel rods, thereby forming the fuel assembly as an integral unit.

Each of the fuel rods within the fuel assembly contains nuclear fuel pellets that are the source of the reactor core's reactive power. All fuel assemblies in the core are substantially the same except that the percent enrichment of fuel material in the fuel rods of one group of fuel assemblies is different compared to other groups of fuel assemblies. be. Traditionally, fuel rods within some of the fuel assemblies in a reactor core have contained enriched uranium, commonly known as fissile material, capable of sustaining chain reactions, while others The fuel rods in a fuel assembly contain natural uranium, commonly known as fuel parent material, which cannot sustain a chain reaction but converts into fissile material upon bombardment with neutrons.

Basically, fuel assemblies containing enriched uranium are known as seed fuel assemblies, while fuel assemblies containing natural uranium are known as blanket fuel assemblies. Various core loading arrangements for seed and blanket fuel assemblies are known in the art. Some such arrangements are described in U.S. Pat.
No. 4096033, No. 4326919 and No. 4378329
No. 1, disclosed in each specification.

In many currently used arrangements, fuel assemblies containing natural uranium, ie, blanket fuel assemblies, are placed in the core buffer to improve conversion of fuel parent materials and reduce neutron leakage. The concept of so-called radial blankets, which are placed directly adjacent to the periphery of the core, has been adopted. However, this positioning of the blanket fuel assemblies relative to the center and periphery of the core has two major drawbacks related to the asymmetric loading of natural uranium that results from such an arrangement. First, the utilization of enriched uranium in the seed fuel assemblies adjacent to the natural uranium in the blanket fuel assemblies is reduced to below the average fuel rate. Second, when these fuel assemblies are shuffled to other locations within the core where natural uranium is no longer present along the core boundaries,
Severe peaking problems can occur.

A primary objective of the present invention is to improve the arrangement of natural and enriched uranium material within the reactor core to increase nuclear fuel efficiency while keeping potential leakage and peaking problems under control.

Accordingly, the present invention resides in a symmetrical blanket fuel assembly that includes a parent fuel material and a fissile material, wherein the parent fuel material and the fissile material are arranged in a symmetrical relationship to each other. Specifically, the fuel parent material, such as natural uranium, is located in the center of the fuel assembly, while the fissile material, such as enriched uranium, is located on all sides of the fuel parent material. This structure ensures leakage and peaking control while providing other significant advantages not available with conventional structures. First, unlike the conventional fuel assembly structure in which only natural uranium is provided on the buttful side of the reactor core, the symmetrical blanket fuel assembly of the present invention
It can be shuffled or repositioned from one core surface to another and from one corner location to another along the core flats without deleterious effects. Moreover, it is also possible to rearrange them within the reactor core while reducing the influence on peaking. Second, by repositioning or shuffling symmetrical blanket fuel assemblies from one core surface to another, the uranium is burned more evenly, which allows the blanket fuel assemblies to last more cycles. It is possible to remain inside the reactor core. Third, by placing the fuel rods containing natural uranium in the water-rich center of the symmetrical fuel assembly, natural uranium can achieve higher burn-up rates. It is also believed that plutonium conversion will be improved. Finally, conventional configurations with natural uranium along the full core require two separate fuel assembly configurations: one with natural uranium along one face and one with natural uranium along two adjacent faces. is required. Therefore, these structures are 90 degrees, 180 degrees of the basic structure.
It must be constructed in four different natural uranium alignment configurations, including degree and 270 degree rotation. In contrast, a symmetrical blanket fuel assembly according to the present invention requires only a single structure without modification.

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: FIG.

In the following description, the same reference numerals refer to the same or corresponding parts throughout the drawings, and include "front", "rear", "left", "right", "upper", "Down" and other similar terminology are used for convenience of explanation and should not be construed in a limiting sense.

Referring now to the drawings, and in particular to FIG. 1, the (blanket) fuel assembly designated generally by the reference numeral 10 is of the type used in pressurized water reactors (PWRs). Basically, the fuel assembly includes a lower end structure or lower nozzle 12 for supporting the fuel assembly on a lower core plate (not shown) of a nuclear reactor and a longitudinal direction from the lower nozzle 12. A plurality of upwardly extending guide tubes or guide thimble 14, a plurality of axially spaced transverse gratings 16 along the guide thimble 14, and a plurality of elongated fuel rods 18 supported by the gratings 16 and laterally spaced apart. It consists of an organized array, an instrumentation tube 20 disposed in the center of the fuel assembly 10, and an upper end structure or nozzle 22 attached to the upper end of the guide thimble 14. With the parts arranged in this manner, the fuel assembly 10 forms an integral unit that can be conveniently handled without damaging the fuel assembly.

As mentioned above, the fuel rods 18 within the fuel assembly 10 are held in spaced apart relationship by grids 16 that are spaced apart along the length of the fuel assembly. Each fuel rod 18 contains nuclear fuel pellets 2
4, both ends of which are closed by end caps 26 and 28.

In the fuel assembly embodying the present invention, some of the fuel rods 18 (first group of fuel rods) contain fuel pellets 24 made of fuel parent material, while other fuel rods contain fuel pellets 24 made of fuel parent material. The rods (second group of fuel rods) contain fuel pellets 24 made of fissile material. This is the only difference between the fuel rods 18. The fissile material generates reaction output, and as the reactor operation progresses, the parent fuel material is converted to the fissile material. A liquid moderator/coolant, such as water or boron-containing water, is pumped upward through the core fuel assembly to extract the heat generated and use it to generate useful work.

The nuclear fission process is controlled by control rods 30 that are reciprocatable within a guide thimble 14 disposed at a predetermined location within the fuel assembly 10 . The upper nozzle 22 includes an internally threaded cylindrical member 3 with a plurality of radially extending pawls or arms 36.
A control mechanism 32 of the rod cluster having 4 is provided. Each arm 36 is configured such that a control mechanism 32 vertically moves the control rods 30 into the guide thimble 14 to thereby control the fission process within the fuel assembly 10, as is well known in the art. It is operatively connected to control rod 30.

As already mentioned, fuel assembly 10 contains natural uranium and enriched uranium material. These materials are provided separately in the form of stacked pellets within individual fuel rods 18. Additionally, each fuel rod 18 containing a different material as described above
They are arranged in a predetermined relationship to enhance the fission process and thereby increase the rate of combustion of the nuclear fuel material.

In FIG. 2, the fuel rods 18 containing the fuel parent material, ie, natural uranium, and forming the first group are represented by a dot "." in the center.
On the other hand, the fuel rods 18 containing fissile material, ie, enriched uranium, and forming the second group are marked with an "x". As can be easily understood from FIG. 2, the first group of fuel rods packed with natural uranium and spaced apart from each other contains fuel rods packed with enriched uranium and spaced apart in a predetermined symmetrical relationship. It is surrounded by a second group of rods.

In a preferred embodiment of symmetry, the fuel rods 1
The first and second groups of eight form a generally square array of fuel rods. In this overall arrangement, the first group of fuel rods forms a centrally located approximately rectangular pattern with four sides, as shown by the interconnected dashed lines in FIG. , the second group of fuel rods
The fuel rods of the group form a generally rectangular pattern arranged on an outer periphery surrounding all four sides. The fuel rods in each group are aligned with each other in rows and columns. Guide thimble 14
are interspersed in spaced relationship between the first and second groups of fuel rods 18. The guide thimbles 14 are also preferably arranged in a generally rectangular pattern so that the guide thimbles are aligned with each other in rows and columns.

Due to the symmetrical relationship of the first and second groups of fuel rods 18, the fuel assembly 10 need not remain in one position or location while within the reactor core. As shown in FIG. 3, during each operating cycle of the core 38, a given fuel assembly within the fuel assembly 10 is It can be shuffled around core buffle 40 so that different sides face core buffle 40 at each location. By relocating from one core surface to another during each cycle, the uranium burns more evenly and fuel utilization is improved.
In addition, by placing the fuel rods loaded with natural uranium in the center of the fuel assembly, which is rich in water,
The fuel is greatly enhanced and the production of plutonium from the fuel parent material is improved.

[Brief explanation of drawings]

FIG. 1 is an elevational view, partially cut away and vertically shortened for clarity, showing a nuclear fuel assembly embodying the present invention; FIG.
An enlarged sectional view of the fuel assembly of FIG. 1, which is an explanatory diagram illustrating the arrangement pattern of the fuel parent material rods, fissile fuel rods, and guide thimble constituting the blanket fuel assembly, and the symmetrical relationship among them; FIG. 3 is a cross-sectional view of a nuclear reactor in which symmetrical blanket fuel assemblies according to the invention are used, illustrating examples of possible positions of the fuel assemblies in successive cycles of the core. 10...Fuel assembly, 18...Fuel rod, 24...
...Nuclear fuel pellets, 38...Reactor core.

Claims (1)

[Claims] 1. In a plumcket fuel assembly for use in a nuclear reactor core that includes a plurality of spaced apart fuel rods, a first group of said fuel rods contains fuel parent material; A blanket fuel assembly, wherein two groups contain fissile material, said second group of fuel rods surrounding said first group of fuel rods in a predetermined symmetrical relationship.